Output data to the user - how?

I'm new to C# (coming from Javascript) and I have a problem. When the user press my button, it hits a function and I want C# to return something to the user - how can I do that?

My script is like this: 

List<object> gruppe = new List<object>();
gruppe.Add (elev[0] + ", " + elev[1]);
gruppe.Add (elev[1] + ", " + elev[2]);

In javascript I wrote something like: 

document.getelementbyid("myDiv").innerText = gruppe[0];